Spectrometers are well known devices for measuring the intensity of light in a beam as a function of wavelengths. A typical spectrometer consists of a slit, a collimating lens, a dispersive optic, such as a prism, grating, or etalon (for angularly dispersing the spectral components according to wavelength), an objective lens or lenses for focusing the various wavelengths and a photometer for measuring the intensity of the various wavelengths. FIG. 1 is a schematic drawing of such a prior art spectrometer using a diffraction grating as a dispersive element. A light source 2 which is the subject of a wavelength measurement is sampled by an optical fiber 4 having an internal diameter of about 250 microns and a portion of the light is directed to slit 6 which is longer than the internal diameter of the fiber and has a width of about 5 microns. Light passing through slit 6 expands in the 5 micron direction (which is in the plane of drawing) in a beam 7 at an angle of about 3 degrees. The beam is reflected from mirror 8 and is collimated by lens 10 for illumination of grating 12 which in this prior art representation is arranged in a Littrow configuration. Light at various wavelengths reflecting from the grating is dispersed at angles dependant on the wavelengths. A beam representing only one wavelength or a very narrow range of wavelengths is depicted in FIG. 1 as reflecting from the grating 12 back through lens 10 and reflecting off mirrors 8 and 14 and is focused to a line at 15. (The long dimension of the line is into and out of the page.) This particular wavelength is refocused at a line 17 by objective lens 16. Light at this wavelength is measured by a photometer 18, while light at other wavelengths is blocked by a slit 19 placed in front of the photometer 18. Slit 19 and photometer 18 are placed in the same housing. Light at wavelengths other than the depicted wavelength (or very narrow range of wavelengths) is reflected off grating 12 at angles slightly different from that of the depicted beam. Thus, other wavelengths are measured at positions above or below line 17 by photometer 18 which, as indicated in FIG. 1, moves back and forth, together with slit 19, to make these intensity measurements. Photometer 18 may be a photo diode array in which case, it could provide a linear spectral representation of a narrow range of wavelengths in the reflected beam. If the light source has an effectively single range of wavelengths as compared to the resolution of the spectrometer, the line image depicted by the photo diode array will be representative of the resolution of that spectrometer.
The resolution of this prior art spectrometer is limited by dispersion of the grating and its size. Both of these parameters can only be improved up to a certain level determined by technology limits and cost. The resolution of the spectrometer can be improved by a factor of 2 by using a double pass arrangement according to U.S. Pat. No. 5,835,210, incorporated hereby by reference. In this arrangement, shown in FIG. 2, a partially reflecting mirror 20 having about 30 percent reflectivity, is inserted between lens 10 and grating 12. The mirror is positioned at a small angle to the beam directed from lens 10 toward grating 12.
The effect of partially reflecting mirror 20 is to reflect about 30 percent of the beam first from grating 12 back again onto grating 12 at a slightly different angle below the direction of the first beam. (About 70 percent of the light in the first reflection transmits partially reflecting mirror 20.) The reflected portion of the beam is reflected and dispersed a second time again returning to partially reflecting mirror where 70 percent of the second reflected beam transmits partially reflecting mirror 20.
This second reflected beam shown as 22 in FIG. 2, goes through lens 10 and reflects off mirrors 8 and 14, and is focused to the line at 24. The beam is refocused at a line 21 by objective lens 16. Light at this wavelength is measured by a photometer 18 in a way similar to FIG. 1. Light at wavelengths other than the depicted very narrow range of wavelengths is reflected off grating 12 at angles different from that of the depicted beam and, therefore, rejected by slit 19 similar to configuration of FIG. 1.
Because of double reflection off grating 12 in this configuration, the angular dispersion of light at different wavelengths will be approximately twice as large as in configuration of FIG. 1.
Therefore, the dispersion of this configuration is improved by a factor of 2, with other components being the same. Persons skilled in the art will recognize, that unfortunately, this scheme still has a disadvantage of having large size. The main limiting factor in size reduction is the focal length of lens 10, which has to be large in order to provide high dispersion. For example, in a prior art spectrometer manufactured by Cymer, Inc. (San Diego, Calif.) lens 10 has a focal length of about 1 m. Other spectrometers might have even larger focal lengths. The beam can be folded using mirrors 8 and 14 as shown in FIGS. 1 and 2 but the large focal length still has quite a limiting effect on size reduction. Another disadvantage is that for good performance, both lens 10 and mirrors 8 and 14 must be of very high optical quality which makes their cost to go up, especially for lens 10, since very high quality lenses can be very expensive.
Therefore, the prior art high quality spectrometer is a bulky and expensive instrument, which restricts their use in most cases to laboratory experiments. There is a need, however, for a high resolution compact spectrometer, which can be used in manufacturing process. A particular need exists for a compact high resolution ultraviolet spectrometer with a resolution of the order of 0.1 pm. Such a spectrometer is needed to monitor the output spectrum of narrow band excimer lasers used in deep UV microlithography. It is very important to make sure that the spectrum of the laser remains line-narrowed during deep UV microlithography. If the spectral bandwidth goes out of specification, the chromatic aberrations may blur the image of electronic components being printed on a silicon wafer and could cause production yield problems.
Therefore, what is needed is a compact spectrometer, which can be built in as a part of a laser or supplied as a field service tool, which is a compact, lightweight and capable of measuring laser spectrum with high resolution, on-line during micro lithographic chip manufacturing.